As the number of services provided by wireless mobile communication devices increases dramatically, so does the need for mobile communications devices that can handle the various forms of signal formats required to provide these services. For example, devices in cellular telephones may need to adhere to standards such as the Global Systems for Mobile Communications (GSM) standard, a Personal Communication Services (PCS), EDGE standard, and Digital Cellular System (DCS) standard. These standards all require precise output power control over a large dynamic range in order to prevent channel interference.
A key component common to mobile telecommunications devices is a radio frequency (RF) power amplification device. Before reaching the power amplification device, the RF transmission signal is too weak for transmission to a cellular base station. Therefore, it is the function of the power amplification device to boost the power of the RF transmission signal.
The power amplification device receives a RF transmission signal that has a constant envelope when the RF transmission signal is being transmitted in accordance with modern Time Division Multiple Access (TDMA) standards, such as GSM standards, and PCS standards. After amplification by the power amplification device, the RF transmission signal must comply with a specification known as a “burst mask.” The burst mask specifies how the mean power of the RF transmission signal should be transmitted in a particular timeslot. More specifically, the burst mask specifies the allowable ramp-up period, duration, and ramp-down period, of the mean power of the RF transmission signal during the timeslot. In a TDMA standard, there may be various and multiple timeslots each having a burst mask specification and the RF transmission signal must conform to the various burst masks during each of the timeslots. If the power amplification device ramps up too slowly, data at the beginning of the burst might be lost, degrading link quality. On the other hand, if the power amplification device ramps up power too quickly, this has the effect of spreading the energy of the RF transmission signal across the spectrum and causing spectrum interference. This is particularly the case when the RF amplification device is being utilized to amplify a RF transmission signal that may be transmitted in both a low transmission band, such as a GSM band and a high transmission frequency band such as a PCS band. One of the major challenges in designing power amplification devices is to be able to meet the high bandwidth requirements while preserving the efficiency of the power amplification device.
Generally, power amplification devices include voltage regulation circuits, such as low-drop-out (LDO) circuits, to provide a regulated voltage to a power amplification circuit that amplifies the RF transmission signal. The LDO circuit generates the regulated voltage from a supply voltage and regulates the regulated voltage level so that fluctuation in the supply voltage level of the supply voltage do not significantly affect the regulated voltage level. This regulated voltage determines the amplification gain of the power amplification circuit. For optimum rated efficiency, the power amplification circuit is driven to operate in saturation by the LDO circuit when the RF transmission signal is a TDMA transmission signal with a constant envelope. However, the LDO circuit should not be driven into saturation because saturation results in significant spectrum interference and a degraded switching spectrum. In essence, the power amplification circuit transitions from the linear region to the saturated region or from the saturated region to the linear region too quickly when the LDO circuit is driven into saturation.
There have been prior designs of LDO circuits configured to soften the transition of the power amplification circuit to and from the linear region and the saturation region. Unfortunately, these designs have significant drawbacks when the power amplification device is designed to amplify RF transmission signals either in a low transmission frequency band or a high transmission frequency band. Increasing the softness of the transition from the saturated region to the linear region reduces a performance of a total radiated power (TRP) of the RF transmission signal. TRP performance is generally of a higher concern during transmission burst within the low transmission frequency band. On the other hand, switching spectrum requirements are harder to meet during transmission burst within the high transmission frequency band. Previous LDO circuit designs provide the softness of the transition as a comprise between providing good TRP performance for RF transmission signals being transmitted within the low transmission frequency band and meeting switching spectrum requirements for RF transmission signal being transmitted within the high transmission frequency band.
Therefore, a need exist for a power amplification device that can provide better performance when the power amplification device amplifies RF transmission signals within in both a high frequency transmission band and a low frequency transmission band.